Does hot QCD have a conformal manifold in the chiral limit?

Abstract

Recent lattice evidence suggests the chiral phase transition in QCD is second-order for $N_f \ge 2$ massless flavors. We constrain CFT descriptions of a critical line in temperature $T$ and imaginary baryon chemical potential $\theta_B = i\mu_B/T$. An 't Hooft anomaly at general $\theta_B$ constrains the transition even at $\theta_B = 0$, leaving only three minimal scenarios. The best-motivated scenario for $N_f\ge3$, and perhaps also $N_f = 2$, is beyond Ginzburg-Landau, featuring a conformal manifold of $\theta_B$-dependent universality classes with an exactly marginal operator related to baryon density.